Conversion coatings on silicon carbide

ABSTRACT

A pack cementation method is described for producing conversion coatings on a ceramic. In particular, silicon carbide is coated with Cr 2  O 3  -forming compounds for improved sulfidation and ash-deposit resistance. The method is applicable to continuous fiber ceramic composites or a monolithic substrate. The conversion coatings are multilayered coating systems with the coating morphologies expressed as follows: 
     
         Cr.sub.23 C.sub.6 /Cr.sub.7 C.sub.3 /Cr.sub.7 C.sub.3 -Cr.sub.3 Si/Cr.sub.5 
    
      Si 3  C x  /Substrate 
     
         Cr/Cr.sub.3 Si/Cr.sub.5 Si.sub.3 C.sub.x /SiC Substrate

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates in general to conversion coatings onceramics, and more particularly to a pack cementation technique toproduce conversion coatings on the surface of silicon carbide (SIC).

2. Description of the Related Art

In the past decade, extensive efforts have been made to develop advancedutility power plants and energy conversion/cogeneration systems.Utilization of these advanced systems may significantly improve energyefficiency and reduce toxic emission. To increase the operationefficiency, however, the utility boilers and conversion/cogenerationsystems must operate at much higher temperatures and steam pressures.Therefore, many components, such as the heat exchanger tubes and hot-gasclean up systems, will be exposed to corrosive environments attemperatures up to 1100°-1370° C. (2000°-2500° F.). These temperaturesare noticeably higher than those experienced in the modern utilityboilers. As a result, suitable materials of construction are critical tothe success of the high-efficiency systems.

While new high-temperature metallic materials are being developed forthese applications, ceramics and ceramic composites are considered theleading candidates to meet the extreme requirements of certain advancedboiler components. Among them, silicon carbide (SIC) is the primeceramic of interest because of its high-temperature properties,including its excellent thermal conductivity (125 W/mK at RT), lowdensity (3.10 g/cm³ for dense material), extremely high mechanicalstrength, relatively good toughness, and low cost. The maximum usetemperature of SiC exceeds 1400° C. (2552° F.).

In general, silicon carbide is corrosion resistant to high-temperatureenvironments. However, SiC has not shown satisfactory corrosionresistance to very sulfidizing (reducing) environments. Under reducingatmospheres, active corrosion may occur on the protective SiO₂ scale dueto the formation of volatile compounds, such as SiO, SiCl₄, SiCl₂, andSiS. The corrosion rates of SiO₂ can be further escalated by molten ashdeposits. When molten ash deposits are present, the SiO₂ formed on thesubstrate surfaces may be readily destroyed via the well-known fluxingmechanisms. Consequently, the underlying SiC substrates will beconstantly exposed to the corrosive environments, and an acceleratedwastage of material is observed.

Unlike SiC, chromium carbides have shown much improved corrosionresistance to sulfidation and molten ash deposit. The corrosionresistance is provided by the formation of a protective chromium oxide(Cr₂ O₃) surface scale. The ability of materials to form a Cr₂ O₃ scalehas served as the basis for the development of numerous heat-resistantalloys and coatings used in severe environments at high temperatures.For example, chromizing coatings have been used to improve theperformance of various boiler components suffering accelerated corrosionattack.

However, chromizing on ceramic materials, such as silicon carbide, itsderivatives, and other types of ceramic carbides, has not been reported.

SUMMARY OF THE INVENTION

The present invention solves the aforementioned problems with the priorart as well as others by providing a pack cementation method forproducing conversion coatings on a ceramic material.

This invention is featured by the formation of conversion coatings onsilicon carbide and its derivatives to improve their high-temperaturecorrosion resistance to sulfidation, chlorination, and molten ashdeposit. The conversion coatings are formed on these carbides consist ofa multi-layered structure, with each of the layers containing a highconcentration of chromium. As a result, the coatings can form aprotective Cr₂ O₃ scale upon exposure to extremely aggressiveenvironments at high temperatures. The superior corrosion resistance ofCr₂ O₃ scale in reducing/sulfidizing environments and ash deposits hasbeen demonstrated by many laboratory and field studies; whereas the SiO₂scale has suffered significantly in similar environments from thecorrosion attack via various corrosion mechanisms proposed in theliterature.

The conversion coating process involves placing the ceramic material ina pack mix inside a steel or ceramic retort. The pack mix comprises asource metal or alloy powder (pure metal powder and/or alloy powder), ahalide salt as activator, and an inert oxide powder as filler. Theretort is sealed and then heated to an elevated temperature in a furnaceand held for an extended period of time under an inert cover gas. At thecoating temperature, the activator reacts with the source metal or alloypowder to form various gaseous halide species. These vapors enable thetransport of the source metal from the pack mix to the substratesurfaces and subsequently, the formation of new source metal-rich phaseson the substrate surfaces.

One object of the present invention is a method for producing conversioncoatings on a ceramic material.

Another object of the present invention is a method for producingconversion coatings on silicon carbide.

Still another object of the present invention is a silicon carbidehaving conversion coatings produced by the method described herein.

The various features of novelty which characterize the invention arepointed out with particularity in the claims annexed to and forming apart of this disclosure. For a better understanding of the invention,its operating advantages and specific objects attained by its uses,reference is made to the accompanying drawings and descriptive matter inwhich a preferred embodiment of the invention is illustrated.

BRIEF DESCRIPTION OF THE DRAWINGS

In the drawings:

FIG. 1 is a ternary phase diagram of the Cr--Si--C system;

FIG. 2 is a cross-sectional optical micrograph of the Hexoloy SA SiC;

FIG. 3(a) shows a cross sectional SEM micrograph of a coated SiC sampleunder the back-scattered electron (BSE) mode;

FIG. 3(b) shows a cross sectional SEM micrograph of a coated SiC sampleunder the secondary-electron (SEl) mode;

FIG. 4 plots the concentration profiles of Cr, Si, and C generated byelectron microprobe analysis (EMPA) from the outer coating surface intothe SiC substrate;

FIG. 5(a) is a cross sectional SEM micrograph of a coated SiC sampleunder the BSE mode;

FIG. 5(b) is a cross sectional SEM micrograph of a coated SiC sampleunder the SEl mode; and

FIG. 6 plots the concentration profiles of Cr, Si, and C generated byelectron microprobe analysis (EMPA) from the coating surface into thesubstrate.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Based on the superior corrosion resistance of chromium carbides andchromium, i.e., their ability to form a more protective Cr₂ O₃ scale,the present invention uses these materials as coatings on SiC. Thechromium-carbide and chromium coatings provide the needed corrosionresistance for SiC exposed to reducing gases and molten ash deposits inadvanced boilers and conversion/cogeneration systems by forming a Cr₂ O₃-base oxide scale. Therefore, the underlying SiC substrate is notdirectly subjected to the severe environments. On the other hand, theSiC substrates still possess the ability to form SiO₂ underneath thechromium-carbide and chromium coatings if defects are developed in thecoating layer. Therefore, a "self-healing" process takes place andallows the surface chromium carbides and chromium to regenerate a Cr₂ O₃protective scale. Consequently, a catastrophic failure is not likely tooccur. An added advantage from using the chromium-carbide and chromiumcoatings is that the difference of thermal expansion coefficients (CTE)between the coatings and SiC is relatively small, thus minimizing thetendency for the coatings to spall or crack during the inevitablethermal cycling in boiler operations.

Because SiC is not electrically conductive, many commercial coatingtechniques, such as ESD and electro-plating, can not be used to applythe chromium-carbide and chromium coatings onto SiC. Other techniquesoperated under the principle of thermal spray often result in a porouscoating morphology and poor coating adherence. The coating method of thepresent invention directly "converts" the surfaces of SiC to chromiumcarbides and/or chromium. This process, also referred to as conversioncoatings, is achieved by chemically "mixing" the coating elements intothe substrate surfaces at high temperatures via diffusional processes.After the coating elements are enriched beyond their solubility limitsin SiC, different phases become stable at the substrate surfaces.Consequently, a surface layer rich in the coating elements is created.Because diffusional processes are involved in the conversion coatingprocesses, the coating adherence accomplished on the underlyingsubstrates is generally superior.

The coatings consist of a Cr₂ O₃ -forming outer layer, such as chromiumcarbides and Cr metal, to provide the needed corrosion resistance byforming a protective oxide scale.

Thermodynamic data show that a total of three chromium-carbide phasesare present in the chromium-carbon binary system, i.e., Cr₃ Cr₂, Cr₇ C₃,and Cr₂₃ C₆. Because both Cr₃ C₂ and Cr₂₃ C₆ (i.e., the lowest andhighest Cr-containing chromium carbides, respectively) are corrosionresistant to sulfidation and ash deposit, as reported in the literature,the intermediate carbide (Cr₇ C₃) should also perform well. However, itis of general consensus that lower carbides, which contain higheramounts of Cr, have a stronger tendency to form protective Cr₂ O₃.Therefore, it is desirable to produce conversion coatings consisting ofan outer Cr₂₃ C₆ and/or Cr₇ C₃ layer. Furthermore, if the metallicchromium phase is formed on the outer surface, the corrosion resistancecan also be significantly improved.

The specific coating parameters generated from the present invention,which successfully produced Cr₂ O₃ -forming conversion coatings systemson SiC, are summarized below. Although these parameters were utilized toproduce the coating layers in the present invention, their ranges can beeasily expanded or the chemicals varied to achieve similar successes.Also, while a monolithic tube is used as the substrate, the method ofthe present invention is equally applicable to fibers. For example, thechromium-carbide and chromium coatings are directly converted on thefibers of SiC and then used to form continuous fiber ceramic composites(CFCCs). Similarly, conversion coatings can be produced directly on thefabricated CFCC.

Coating Temperature: 1250° C. (2282° F.)

Coating Time: 30 Hours

Cover Gas: De-oxidized Argon

Pack Composition (wt. % ) 20% Source Metal or Alloy (Cr or Cr--Al)

3% Activator (AlCl₃ or NH₄ Cl)

77% Inert Filler (Al₂ O₃)

Using the above coating parameters, a total coating thickness of 160-185μm is achievable on SiC and its derivatives. The coating layers arerelatively dense and pore-free compared to the underlying SiC substrate.Excellent coating adherence is accomplished. The coating systems containa multi-layered structure with different layers consisting of differentphases. The coating systems produced by using AlCl₃ and NH₄ Cl as theactivator are different, i.e.,:

AlCl₃ Activator: Cr₂₃ C₆ /Cr₇ C₃ /Cr₇ C₃ +Cr₃ Si₃ Si₃ C_(x) /SiCSubstrate

NH₄ Cl Activator: Cr/Cr₃ Si/Cr₅ Si₃ C_(x) /SiC Substrate

Although there is extensive experience in chromizing of carbon andlow-alloy steels for boiler replacement parts, the coating art islimited to metallic substrate materials. Using the pack cementationtechnique to form conversion coatings on ceramic materials must beconducted at a much higher temperature than those employed for thediffusion coatings on metals. The higher processing temperatures cansignificantly alter the chemical equilibria and predominant vaporspecies in the coating packs, which then affect the reaction kineticsinvolved in the coating mechanisms. Unlike diffusion coatings on metals,a diffusion zone, in which a concentration gradient of the coatingelements is observed, is generally not existing for ceramics.Furthermore, other considerations should be stressed for diffusioncoatings on ceramics, such as the requirement for an optimal match ofthermal expansion coefficients between the coating materials and the SiCsubstrates subjected to thermal cycling.

The pack-cementation coating process involves burying the parts to becoated with a pack mix in a retort. The pack mix is comprised of powdersof (1) a source metal or alloy (masteralloy alloy), (2) a small amountof halide salt (activator), and (3) a large quantity of inert oxide(filler). The retort is sealed and then heated to an elevatedtemperature in a furnace and held for an extended period of time. At thecoating temperature, the activator reacts with the masteralloy to formvarious gaseous species of metal halides in the pack. Because of thepresence of an activity gradient of the coating element(s) between thepack mix and the substrate, the vapors of lower halides (i.e., thosespecies containing higher ratios of the coating elements to halogen)tend to diffuse toward the substrate surfaces through the pores of thepack mix. At the substrate surfaces, reduction reactions take place andthe coating elements are released from the metal halides. The coatingelement(s) then mixes with the substrate and becomes part of thesubstrate constituents in the surface regions. Details of metal packcementation processes and coating mechanisms are known in the art andare not repeated here.

To achieve chromium-carbide and chromium conversion coatings on SiC, Crmust be introduced to the SiC surfaces via the diffusional processes athigh temperatures. Therefore, Cr metal or a Cr-containing alloy needs tobe employed as the source metal (masteralloy). Many halide salts canpotentially be used as the activator in the pack coating processes.However, a proper selection of the activator salt is imperative to thesuccess of achieving conversion coatings on SiC.

For successful conversion of the SiC surfaces to chromium carbidesand/or chromium the coating phases must be more thermodynamically stablethan those of SiC and Cr in direct physical contact. The higher tendencyfor the formation of chromium carbides was demonstrated by Pellegrini etal., "A Survey of the Cr-Rich Area of the Cr--Si--C Phase Diagram," J.Electrochem, So., Vol. 1991, No. 4, p. 535, 1972 who experimentallydetermined the ternary phase diagram of the Cr--Si--C system at 1400° C.(2552° F.), shown in FIG. 1. Lyakishev et al., "ThermodynamicInvestigation of Inter-Particle Interactions in the Me [Ti, V, Cr, Mn,Fe, Co, Ni]--Si--C Systems as Theoretical Prerequisites for Improvingthe Technology of Smelting Bulk Silicon Ferroalloys," Russian Metallurgy[1], 1-9, Jan.-Feb., 1991, also constructed the Cr-Si-C phase diagram atdifferent temperatures based on thermodynamic calculations, and theirresults were essentially identical to those shown in FIG. 1. Therefore,FIG. 1 is representative of the Cr--Si--C phase diagram in a widetemperature range.

The tie lines for the two-phase equilibria in FIG. 1 are connectingchromium carbides and chromium silicides, such as Cr₇ C₃ with Cr₃ Si andCr₃ Si₃ C_(x). Such a relationship clearly demonstrates that thecombination of a chromium carbide and chromium silicide is more stablethan that of SiC and the Cr source metal. In other words, it is notthermodynamically stable for SiC and the Cr source metal to be in directcontact, and they must be separated by the formation of chromiumcarbide(s) and chromium silicide(s). A metallic Cr phase may form whenthe Cr activity is extremely high.

The phase diagram also signifies that, when chromium carbides are formedin between SiC and Cr, chromium silicides can also be formed as part ofthe coatings. The co-existence of chromium silicides with chromiumcarbides in the coating layers is desirable because of an improvedmechanical properties resulting from the existence of a dual-phase (orcomposite) structure. In addition, both Cr and Si in chromium silicidescan form protective oxides, so long as the chromium silicides are richin Cr to ensure the formation of Cr₂ O₃.

EXAMPLES

A one-foot segment of Hexoloy SA alpha-SiC tube with a dimension of 4"OD×1/8" wall thickness was obtained. The monolithic SiC tube wasproduced by Carborundum, Niagara Falls, N.Y., by pressureless sinteringof submicron SiC powder. The sintering process resulted in aself-bonded, fine grain (less than 10 micron) SiC product which isextremely hard, light weight, and relatively low in porosity. Table 1lists some of the important physical and mechanical properties of theHexoloy SA alpha-SiC. FIG. 2 is a cross-sectional optical micrograph ofthe SiC material containing a large amount of preexisting fine porosity.

                  TABLE 1                                                         ______________________________________                                        Typical Physical and Mechanical Properties of Hexoloy SA                      ______________________________________                                        SiC                                                                           Grain Size           4-6        μm                                         Density              2.99-3.10  g/cm.sup.3                                    Porosity             1.1%                                                     Hardness (Knoop)     2800       Kg/mm.sup.2                                   Flexure Strength (4 Pt., RT)                                                                       460        MPa                                           Flexure Strength (3 Pt., RT)                                                                       550        MPa                                           Compressive Strength (RT)                                                                          3900       MPa                                           Elastic Modules (RT) 410        MPa                                           Fracture Toughness (RT)                                                                            4.60       MPa/m1/2                                      Coeff Thermal Expansion (RT-700° C.)                                                        4.02 × 10.sup.-6                                                                   1/°K.                                  Maximum Service Temp. (in Air)                                                                     1650°                                                                             (3000° F.)                             ______________________________________                                    

In the present study, pure Cr metal powder and a 90Cr-10Al alloy powderwere chosen as the source metals (masteralloys). Pure Cr powder isreadily available commercially and therefore is an ideal raw material tobe used in large-scale coating productions. The incorporation of90Cr-10Al as a masteralloy was to evaluate the effects fromco-deposition of Al with Cr on the resulting chromium-carbide andchromium coatings.

Five halide salts, including AlF₃, NH₄ Cl, AlCl₃, NaCl, and NaF, wereinvestigated in this study. The partial pressures of various vaporspecies in the packs containing pure Cr as the masteralloy inequilibrium with these activators have been previously extensivelyevaluated, and the results serve as the basis of the system designshere. Alumina powder (Al₂ O₃) was selected as the inert oxide fillerbecause of its extremely high chemical stability, high melting point,excellent commercial availability, and low cost. Table 2 summarizes thechemical constituents of the six pack-mix systems designed for thisconversion-coating study.

                  TABLE 2                                                         ______________________________________                                        Chemical Constituents of the Designed Coating Systems                         System  Masteralloy    Activator  Inert Filler                                ______________________________________                                        1       Cr             AlF.sub.3  Al.sub.2 O.sub.3                            2       Cr             NH.sub.4 Cl                                                                              Al.sub.2 O.sub.3                            3       Cr             AlCl.sub.3 Al.sub.2 O.sub.3                            4       Cr             NaCl       Al.sub.2 O.sub.3                            5       Cr             NaF        Al.sub.2 O.sub.3                            6       90Cr--10Al     NaF        Al.sub.2 O.sub.3                            ______________________________________                                    

All of the coating systems in Table 2 consisted of 3 wt % activator, 20wt % source metal (masteralloy), and 77 wt % Al₂ O₃. Constituents of thecoating packs were carefully weighed and thoroughly mixed. Coupons ofSiC samples with a dimension of 1/2"×3/4×1/4" were cut from the one-foottube segment and used as the substrates. The SiC coupons were cleaned inmethanol and dried; no further surface polishing and cleaning wereemployed. The SiC samples were buried in 1" OD×3" alumina cruciblesserving as the coating retorts with the pack mixes. The aluminacrucibles were then covered with alumina disks and sealed with aluminacement (Ceramobond).

Following the assembly of the coating retorts, the crucibles werepositioned in the center of an alumina reaction chamber (5" OD×4' long)located in a vertical high-temperature tube furnace. The furnace,manufactured by Ohio Thermal Inc., Columbus, Ohio, was equipped withMoSi₂ heating elements. The maximum temperature capability of thisfurnace was 1600° C. (2912° F.). The end of the reaction chamber wascovered with stainless-steel flanges which contained fittings to housethe necessary penetrations for the gas inlet/outlet openings andthermocouples.

All of the conversion-coating processes in Table 2 were conducted at1250° C. (2282° F.) for 30 hours. Table 3 summarizes the coatingparameters employed in this study. Industrial-grade argon gas (Ar) waspurged through the reaction chamber and used as the cover gas tominimize oxidation of the Cr source metal and 90-Cr-10Al master alloy.Prior to entering the reaction chamber, the Ar was further purified bypassing it through an oxygen getter containing titanium chips heated at700° C. (1400° F.).

                  TABLE 3                                                         ______________________________________                                        Coating Parameters for All Coating Systems                                    ______________________________________                                        Coating Temperature:                                                                              1250° C. (2282° F.)                         Coating Time:       30 hours                                                  Cover Gas:          De-oxidized Ar                                            Pack Composition (wt. %)                                                                          20% Source Metal                                                              3 Activator                                                                   77% Al.sub.2 O.sub.3 Filler                               ______________________________________                                    

The appearances of the as-coated SiC samples treated with the sixconversion-coating systems (Table 2) were quite different. The samplefrom System #1 did not provide any evidence of successful surfacecoating; the sample retained the original black SiC color. Compared tothe initial dimension of the SiC substrate prior to the coatingtreatment, this coupon actually displayed some material loss. A loss inmaterial suggests that chemical attack from the pack mix has occurred.It is most likely that the SiC surfaces reacted with the activator(AlF₃) to form a low-melting compound at the coating temperature. A lossof material was only observed on the samples coated in System #1.

Unlike System #1, the resulting coupon surfaces from Systems #2 and 3exhibited a shining appearance, similar to those of finished metals.This appearance strongly suggested that some sort of coatings have beensuccessfully achieved.

Visual inspection on the coatings from Systems #2 and 3 indicated thatthe coating adherence was good; no evidence of coating spallation orsurface cracking was found macroscopically. Unlike System #1, thesesamples did not suffer from any chemical attack from the pack mixes. Infact, the sample dimension was slightly increased, indicating asuccessful incorporation of the coating element (i.e., Cr) onto thesubstrates by these coating processes.

The appearance of the as-coated SiC samples from Systems #4-6 weresomewhat similar. All of which showed some degree of success inachieving a surface coating; however, spallation was also evident onthese coated samples. The mode of coating spallation appeared to belocalized along the edges of the samples coated from System #6, but thespallation was randomly distributed on the sample surfaces treated inSystem #4. Both of the spallation modes were found on the coated samplesfrom System #5. Comparing these three coated SiC, conversion coatingsproduced by System #6 seemed to be the best, followed by System #4 andSystem #5. In comparison, the coating quality produced by Systems #4-6was between those of System #1 and Systems #2 and 3.

It is noted that the same activator salt (NaF) but different sourcemetals were employed in Systems #5 and 6. A better coating qualityachieved by System #6 than System #5 implies that some beneficialeffects from the incorporation of Al to the masteralloy do exist.

FIGS. 3(a) and (b) show two cross-sectional SEM micrographs of the SiCsamples coated with System #3. Both micrographs were taken at the samelocation, except that one was under the secondary-electron (SEl) mode(FIG. 3b) to reveal the coating geometric profile and the other wasunder the back-scattered electron (BSE) mode (FIG. 3a) to highlight thecompositional profile.

The SEl micrograph in FIG. 3(b) shows that a total coating thickness of˜160 μm (6.3 mils) was achieved on SiC. The adherence between thecoating and the underlying SiC substrate is excellent. No evidence ofany separation or spallation of the coating was found. The coatingcontains some voids. It is noted that the Hexoloy SA SiC substratesemployed here contains pre-existing porosity. It is likely that, duringthe conversion coating treatment, the large number of small voidspre-existing in the substrates were annihilated to form a fewer butlarger voids, perhaps via a growth mechanism similar to that of particlecoarsening in metals, often referred to in the art as Ostwald ripening.

However, the voids did not affect the coating adherence in any way, asevidenced by the good coating adherence in FIG. 3. The majority of thevoids are confined close to the coating/substrate interface. In fact,the coating near the outer surface is relatively dense and pore-free. Adense and pore-free outer layer is highly desirable for coatings appliedon porous substrates, because they can seal the surfaces and preventcorrodants from entering the underlying substrate pores.

The sharp contrast present in the BSE micrograph of FIG. 3(a) clearlyindicates that the coating contains three distinctive phase layers withabrupt compositional changes. The brightnesses of the coating layers aredirectly proportional to their average atomic weights, i.e., the higheratomic weight a coating layer is, the brighter the layer is under theBSE mode. In the Cr-Si-C system, Cr has the highest atomic number,followed by Si and C. Therefore, the outermost layer with a thickness of˜20 μm consists of the highest Cr concentration, followed by the middleand the inner coating layers, each of which at a thickness of ˜70 μm.The adherence at each of the phase transition interfaces between twolayers is also extremely good.

Careful examinations of the BSE micrograph in FIG. 3(a) reveal that themiddle layer actually consisted of two sub-layers, with the region closeto the outermost layer rich in a "darker" phase and the region adjacentto the inner layer rich in a "lighter" phase.

FIG. 4 plots the concentration profiles of Cr, Si, and C from the outercoating surface into the SiC substrate. The elemental concentrationswere obtained using the electron microprobe analysis (EMPA). The totalcoating thickness, the three sub-layers, and the points of phasetransitions are clearly displayed by these concentration profiles. Inthe outer layer, the coating is comprised of ˜90 wt. % Cr, >7 wt. % C,and essentially no Si. This compostion indicates that a binarychromium-carbide typical for Cr₂₃ C₆ was formed. In the middle layer,the coating composition is divided into two parts, as suggested by thecross-sectional BSE micrograph in FIG. 4. The first half is close to theouter surface and has a thickness of ˜30 μm. The composition of thisregion consisted of ˜85wt. % Cr, >7 wt. % C, and essentially no Si.Again, this composition suggests that a binary chromium-carbide phasewas produced. The relative concentrations of Cr and C are typical of Cr₇C₃.

The second half of the middle layer, adjacent to the inner layer, has atotal thickness of ˜40 μm and contains a Cr concentration fluctuatingfrom ˜82 to 85 wt. %, C from 5 to 10 wt. %, and Si from 0 to 15 wt. %. Afluctuation in the coating composition confirms the presence of atwo-phase structure in the second half of the middle layer, as suggestedby the BSE SEM micrograph in FIG. 3(a). The phase containing no (or verylittle) Si in this region is still Cr₇ C₃. The other phase consisting of˜15 wt. % Si in equilibrium with the Cr₇ C₃ compound, based on theCr-Si-C ternary phase diagram in FIG. 1, should therefore be Cr₃ Si, abinary Cr-Si intermetallic compound.

FIG. 4 shows that the inner (third) layer, exhibiting the lowestbrightness within the coating in the BSE micrograph (FIG. 3[a]),contains ˜71 wt. % Cr, >7 wt. % C, and 24wt.% Si. Similar to the outerlayer, the chemical composition in the inner layer is quite uniform,suggesting that it is comprised of a single phase. However, unlike theouter layer, this layer has significant amounts of all three elements,which suggest that this is a ternary compound. Based on the phasediagram in FIG. 1, the only ternary compound existing in the Cr-Si-Csystem is the T phase with a formula of Cr₅ Si₅ C_(x), where 0.25<×<1.05at 1400° C. The range of x may change at different temperatures.

Some cracks perpendicular to the sample surface, such as the one shownon the upper right-hand side of the SEM micrographs in FIGS. 3(a) and(b), were found in the coating. The cracks were closely associated withthe "columnar" grain boundaries in the conversion coating. A columnargrain structure is a feature of diffusion coatings resulting frominward/outward diffusion of atoms perpendicular to the substratesurfaces. The cracks found in this conversion coating extended typicallyfrom the outer surface of the coating to the middle-layer/inner-layerinterface.

The cracks in the coating might have been caused by the slight CTEmismatch (i.e., difference in the coefficients of thermal expansion)between the conversion coatings and the substrate. It is most likelythat the crack was created during the sample preparations. However, thecracks did not penetrate throughout the coating layer; instead, theyterminated at the front of the T phase. Therefore, the porous SiCsubstrate will still be protected by a high Cr-containing, potentiallyCr₂ O₃ -forming compound. For the purpose of corrosion resistance, theexistence of vertical cracks would be more favored than that of parallelcracks along the coating/substrate interface. Parallel cracks tend tosignificantly deteriorate the coating adherence, while vertical crackspreserve the coating integrity. In addition, protective oxides, such asCr₂ O₃ and glassy SiO₂, may still form within the vertical cracks andtherefore, can minimize the localized corrosion attack within thecracks.

The multi-layered coating morphology resulting from System #3 can beexpressed as:

    Cr.sub.23 C.sub.6 /Cr.sub.7 C.sub.3 /Cr.sub.7 C.sub.3 +Cr.sub.3 Si/Cr.sub.5 Si.sub.3 C.sub.x /SiC Substrate

FIGS. 5(a) and (b) show two cross-sectional SEM micrographs of the SiCsample coated with System #2. Again, both micrographs were taken at thesame location but one under SEl mode (FIG. 5[b]) and the other BSE mode(FIG. 5[a]). The SEl micrograph reveals that a total coating thicknessof 160-185 μm was achieved. Similar to the samples coated in System #3,voids are found in the coating as a result of the pre-existing substrateporosity. Most of the voids are also distributed near thecoating/substrate interface, and the outer surface of the coating isrelatively dense and essentially pore-free. However, compared to thecoating from System #3, this coating appears to contain relatively morevoids and the outer coating surface is not as uniform.

The BSE micrograph (FIG. 5a) shows that, like the coating from System#3, this coating also exhibits a multi-layered structure consisting ofthree layers.

FIG. 6 summarizes the concentration profiles of Cr, Si, and C generatedby EMPA from the coating surface into the substrate. The outer layer,with a thickness of ˜20 μm, contains 90-96 wt. % Cr, 2.5-3.6 wt. % Si,and ˜1 wt. % C. Such a high Cr concentration suggests that the outermostlayer is the Cr metal phase containing small amounts of dissolved Si andC. The middle layer is composed of ˜85 wt. % Cr, 15.5% Si, and 2 wt. %C. According to the ternary phase diagram in FIG. 1, the middle layerwould be the Cr₃ Si intermetallic compound containing some dissolved C.FIG. 1 shows that an equilibrium between Cr₃ Si and the Cr metal phaseis possible.

The composition of the innermost layer is comprised of ˜73 wt. % Cr,24wt. % Si, and 4-6 wt. % C. Again, using the phase diagram in FIG. 1,the layer containing an appreciable amount of C in equilibrium with Cr₃Si should be the T phase, i.e., the Cr₅ Si₃ C_(x) compound.

Overall, the multi-layered coating morphology achieved by System #2 canbe expressed as:

    Cr/Cr.sub.3 Si/Cr.sub.5 Si.sub.3 C.sub.x /SiC substrate

All of the Cr, Cr₃ Si, and Cr₅ Si₃ C_(x) sub-layers converted on SiCcontain high concentrations of Cr. Thus, similar to the chromium-carbidecoating achieved by System #3, they all possess the ability to form aprotective Cr₂ O₃ scale on SiC upon exposure to corrosive environments.

Two multi-layered conversion coating systems, with total coatingthicknesses of 160 and 160-185 μm, respectively, were produced onHexoloy SA SiC. The resulting coating morphologies are expressed asfollows:

    Cr.sub.23 C.sub.6 /Cr.sub.7 C.sub.3 /Cr.sub.7 C.sub.3 +Cr.sub.3 Si/Cr.sub.5 Si.sub.3 C.sub.x / SiC Substrate

    Cr/Cr.sub.3 Si/Cr.sub.5 Si.sub.3 C.sub.x /SiC substrate

Both coating systems exhibit excellent coating adherence. In addition,the outer surfaces of the coatings are relatively dense and pore-free.Such a morphological feature allows the surface coatings to effectivelyprotect the porous substrate materials from the corrodants. Theinclusion of Al to the source metal (masteralloy) in the pack mixappears to improve the coating quality, as demonstrated by System #6.Such findings suggest that co-deposition of other elements with Cr mayfurther enhance the characteristics of the conversion coatings. It ispossible that co-diffusion of multiple elements may be engineered toimprove the coating toughness and eliminate surface cracks. It ispossible to co-diffuse other elements, such as Al, with Cr in conversioncoatings processes to further improve the coating adherence, morphology,and other mechanical properties. For example, the addition of Al to themasteralloy of System #6 may have increased the coating toughnessslightly.

It is believed that the voids formed in the coating layers are primarilycaused by the large number of pre-existing pores in the SiC substrates.Therefore, to minimize the void formation, it is necessary to select aSiC material with a higher bulk density.

Another aspect of the present invention is directed towards producing ahigh temperature coating converted directly on the continuous fiberceramic composites (CFCC) fibers and within their surface pores toimprove the corrosion resistance of the SiC-based CFCCs.

The present invention produces conversion coatings on the fibers andpore surfaces of CFCCs. These coatings improve the corrosion resistanceand gas tightness of the CFCCs. The chromium-carbide and chromiumcoatings are directly converted on the fibers and the pore surfaces ofthe CFCC like silicon carbide CFCC via the previously describeddiffusion process. Because surface reactions and diffusion are involved,the resulting conversion coatings exhibits excellent adhesion to the SiCsubstrate surfaces. The existence of a chromium carbide or chromiumouter layer enables the formation of a protective Cr₂ O₃ scale on thesurfaces of carbide-based CFCCs upon exposure to high temperaturecorrosive environments. Thus, the corrosion resistance improvescarbide-based CFCCs to molten ash deposit and sulfidation. In addition,the resulting volume expansion of the fibers from the coating formationnarrows or closes the gaps of the porosity making them suitable forapplications where gas tightness is required.

The pack cementation technique utilized in conversion coatings processesof the present invention is quite cost effective compared to other typesof commercial coating techniques. All of the coating chemicals requiredare readily available commercially. The coating processes are alsorelatively simple and not labor intensive.

In large-scale production of the conversion coating on SiC and itsderivatives, the commercial chromizing facility existing for coatingmetallic substrates may be used, and only minimum efforts are requiredfor equipment modification.

The conversion coating systems invented here contain a multi-layeredstructure, with the innermost layer being more ceramic like and theoutermost layer more metal like. Such coating morphologies offer agradual transition in the coefficients of thermal expansion (CTE).Therefore, the CTE mismatch between the coating layers and the SiCsubstrate would be minimized. A low CTE mismatch should result in abetter coating adherence under thermal cyclic conditions. The additionof Al to the pack mix (or coating systems) may increase the coatingadherence.

While specific embodiments of the present invention have been shown anddescribed in detail to illustrate the application and principles of theinvention, it will be understood that it is not intended that thepresent invention be limited thereto and that the invention may beembodied otherwise departing from such principles.

Although the process parameters for conversion coatings of siliconcarbide are specified, their ranges can be expanded or the chemicals bevaried to achieve similar successes. For example, based on simplediffusion analysis, similar coating results may be achieved in thecoating temperature range of 1000°-1400° C. with proper variation in thecoating times. The cover gas may be either industrial argon, nitrogen,or other inert and reducing gases, with or without a prior de-oxidationtreatment. The source metal/master alloy may be powder of pure Cr orCr-Al alloy containing up to 20 wt. % Al. The activator may be theclaimed AlCl₃ and NH₄ Cl, or other inorganic salts, such as AlBr₃ andNH₄ Br. The Al₂ O₃ inert filler used here may be easily replaced byother stable oxides, such as ZrO₂ CaO, MgO and SiO₂ (Note: SiO₂ may beused at lower coating temperatures so that it will not sinter). Inaddition, the same coating concept and process can be used to producechromium-rich coating phases on many other types of ceramicmaterials/compounds, such as boron carbide (B₄ C), silicon nitride (Si₃N₄), and titanium carbide (TIC).

I claim:
 1. A method for making a chromium carbide conversion coating ona ceramic carbide, comprising the steps of:placing the ceramic carbideand a pack mix in a retort; providing an inert cover gas in the retort;heating the retort to a temperature in the range of 1000°-1400° C. todiffuse chromium into a surface of the ceramic carbide; and maintainingthis temperature for a period of time to convert the surface of theceramic carbide to a multi-layer coating which includes chromiumcarbide.
 2. A method as recited in claim 1, wherein the pack mixcomprises at least one metal source, at least one activator, and aninert filler.
 3. A method as recited in claim 2, wherein the at leastone metal source is a member selected from the group consisting of apure metal powder, and an alloy powder.
 4. A method as recited in claim3, wherein the at one least metal source is a member selected from thegroup consisting of chromium and chromium-aluminum.
 5. A method asrecited in claim 4, wherein the pack mix includes a compositioncomprising on a weight percent basis of about 20 percent metal source,about 3 percent activator, and about 77 percent inert filler.
 6. Amethod as recited in claim 5, wherein the heating step includes the stepof heating to about 1250° C. and holding at that temperature.
 7. Amethod as recited in claim 6, wherein the holding step includes the stepof holding at about 1250° C. for about 30 hours.
 8. A method as recitedin claim 2, wherein the at least one activator is a member selected fromthe group consisting of AlCl₃, NH₄ Cl, AlBr₃, and NH₄ Br.
 9. A method asrecited in claim 2, wherein the inert filler is a member selected fromthe group consisting of Al₂ O₃, ZrO₂, CaO, MgO, and SiO₂.
 10. A methodas recited in claim 1, wherein the ceramic carbide is a member selectedfrom the group consisting of silicon carbide, boron carbide, andtitanium carbide.
 11. A method for forming a chromium carbide conversioncoating on silicon carbide, comprising the steps of:placing at least onesurface of a silicon carbide workpiece in a pack mix in a retort;providing an inert cover gas in the retort; sealing the retort; heatingthe retort to a temperature in the range of 1000°-1400° C. to diffusechromium into the at least one surface of the silicon carbide workpiece;and maintaining this temperature for a period of time to convert the atleast one surface of the silicon carbide workpiece to a coating whichincludes chromium carbide.
 12. A method for forming a chromiumconversion coating on silicon carbide, comprising the steps of:preparinga pack mix comprising a chromium-containing source metal, at least oneactivator, and a filler; placing at least one surface of a siliconcarbide workpiece in the pack mix in a retort; providing an inert covergas in the retort; sealing the retort; and heating the retort to atemperature of about 1250° C. and holding at that temperature to diffusechromium into the at least one surface of the silicon carbide workpieceto convert the at least one surface of the silicon carbide workpiece toa coating which includes chromium carbide.
 13. A method as recited inclaim 12, wherein the heating step includes the steps of holding at atemperature of about 1250° C. for about 30 hours.
 14. A method asrecited in claim 12, wherein the at least one activator is a memberselected from the group consisting of AlCl₃, NH₄ Cl, AlBr₃ and NH₄ Br.15. A method as recited in claim 12, wherein the pack mix includes acomposition on a weight percent basis of about 20 percent chromiumsource metal, about 3% activator, and about 77% inert filler.
 16. Amethod as recited in claim 15, wherein the inert filler is a memberselected from the group consisting of Al₂ O₃, ZrO₂, CaO, MgO, and SiO₂.17. A method as recited in claim 12, wherein the chromium-containingsource metal is a member selected from the group consisting of a purechromium metal powder and a binary chromium-aluminum powder.